ABSTRACT Malaria is caused by the protozoan parasite, Plasmodium. It begins with the infection by Plasmodium sporozoites of the liver. This step is essential for the expansion of parasite numbers and the subsequent symptomatic erythrocytic cycle. Dormant liver stages formed by P. vivax are the major cause of malaria relapses. Therefore, inhibition of pre-erythrocytic infection will prevent malaria pathology and relapses from P. vivax. Current drugs against pre-erythrocytic stages have significant side-effects or are expensive. Therefore, there is an urgent need for new drugs against pre-erythrocytic stages. We propose to initiate a medicinal chemistry effort to optimize an inhibitor of P. falciparum's cGMP-dependent protein kinase (PKG). PKG is essential for sporozoite invasion of hepatocytes and subsequent development in liver stages. Its chemical inhibition by a trisubstituted pyrolle (TSP) prevents liver infection in tissue culture assays and in mice. We hypothesize that optimization of TSP's physical properties will yield compounds with low dose efficacy against P. berghei liver stages. Our collaborative work combines expertise and experience in pre-erythrocytic stage biology and kinases, medicinal chemistry of kinase inhibitors and computational drug design. Our specific aims are: Specific Aim 1: Design and synthesize novel TSP analogs. Medicinal chemistry techniques guided by P. falciparum PKG (PfPKG) X-ray crystal structure will be used to synthesize analogs predicted to have improved potency and permeability properties. Specific Aim 2: Determine in vitro potency and liver stage activity of TSP analogs. Potency against PfPKG enzyme, whole-cell activity against P. falciparum erythrocytic stages and P. berghei sporozoite infection of HepG2 cells, ADME, toxicity and specificity studies will be considered in selecting parasite-selective compounds for testing in Aim 3 Specific Aim 3: Determine in vivo efficacy of TSP analogs. Compounds (single concentration, multiple dosing) will be tested through oral and intravenous administration to mice infected with luciferase-expressing P. berghei sporozoites. Liver stage infection will be quantified using luminescence measurements. Compounds that significantly inhibit liver parasitemia are likely to possess reasonable pharmacokinetic and pharmacodynamics properties, appropriate for future optimization.